Pulsed Laser Deposition of Superconductor/Ferromagnetic YBa 2 Cu 3 O y /SrTiO 3 /La 2/3 Sr 1/3 MnO 3 Heterostructures
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ABSTRACT YBa 2 Cu 3Oy/SrTiO 3/La2/ 3Srl/3MnO 3 heterostructures have been deposited on LaAIO 3(001) and SrTiO 3(001) substrates by pulsed laser deposition. First, the influence of deposition conditions on crystallinity and morphology of single LSMO films was examined. Results were used for preparation of heterostructures in tri-layer and cross-strip geometry. Cross-strip geometry was defined by direct shadow mask patterning. Different characterization techniques have been used to determine and correlate the heterostructure properties. A complete analysis of the crystal structure has been carried out with a four-circle difractometer. Morphology has been studied by scanning electron microscopy and atomic force microscopy in order to determine surface roughness and droplet density. Basic electrical properties of films have been determined. INTRODUCTION Trilayers of superconductor-insulator-ferromagnet present a great interest in the development of new electronic spin devices based on oxides, such as spin transistors, memories, and sensors. Spin injection devices are based on the fact that the injection of a spin-polarized current from a ferromagnet can strongly suppress supercurrent due to the Cooper pair breaking. As the source of polarized quasiparticles can be used colossal magnetoresistive (CMR) manganites such as La 2/3Sr 1 /3Mn0 3 (LSMO). SrTiO 3 (STO) seems to be suitable chemical and
tunneling barrier material due to low lattice mismatch with LSMO and high-temperature superconductor YBa 2Cu 3Oy (YBCO). The architecture of the device to obtain a good performance has a substantial complexity. Sharp magnetic and electric interfaces between thin films are important in order to avoid scattering of spin polarized quasiparticles which can decrease efficiency of a spin injection device. In addition, the tunneling effect is limited only to barriers thinner than 10 nm [1]. Therefore, the interface roughness plays an important role. Particularly, the droplets which are typical of pulsed laser deposition should be avoided. The films of the trilayer have to be grown epitaxially and to maintain the correct stoichiometry to have the correct functional performances. Properties of the three commonly used materials depend strongly on the oxygen stoichiometry. The very high electrical resistivity of STO drops notably with the presence of oxygen vacancies.
LSMO as well as YBCO needs of a fully-oxidized state to present the required properties. So, special care is necessary to ensure the correct oxygen stoichiometry when the trilayer is prepared. Finally, the simplest and most reproducible technique to prepare the small junction area required in the device architecture is desirable. Due to this complexity, only a few groups have successfully prepared these devices during the last few years, using rather complicated photolithography processes involving wet-etching or ion-milling for trilayer junction patterning [2-4]. Both experiment and theory are in early stages of development.
335 Mat. Res. Soc. Symp. Proc. Vol. 574 © 1999 Ma
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